The present invention relates to electronic fuel injection systems for internal combustion engines and in particular to an electronic control circuit for controlling the energization of electromagnetic fuel injection valves.
With the ever increasing demands on fuel economy and emissions control for vehicles, the application of electronically controlled fuel delivery systems has become increasingly prevalent. Recently, with the advent of powerful, inexpensive microprocessors, these controllers have become highly sophisticated, monitoring and/or precisely controlling such engine parameters as fuel-air ratio, ignition timing, ambient air and exhaust gas temperatures, oxygen in the exhaust system, etc. Particularly in the area of emissions control, current technology relies on extremely precise control of fuel-air ratio. For example, three-way catalysts have a fuel-air ratio "window" of only approximately one percent within which they operate reasonably efficiently on all three emission components; namely hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). Excursions of the fuel-air ratio outside this window not only result in low conversion efficiency during the excursion, but also reduce the operating temperature of the catalyst thereby resulting in a loss of conversion efficiency for a period after the fuel-air ratio has been brought back within the window.
In throttle body injection systems, the metering of fuel is controlled by an electromagnetic injection valve. A fuel injector typically comprises a precise orifice which is connected through a solenoid valve to a source of pressurized fuel. The valve is actuated via energization of the solenoid coil by a pulsed electrical signal characterized by a pulse width (PW) and a frequency (f). The amount of fuel delivered is thus given by the formula:
Fuel Flow=PW.times.f.times.C;
wherein C is a constant determined in accordance with the size of
the orifice and fuel pressure.
Because pulse widths can be as small as a millisecond in typical applications, the mechanical delay in the operation of the solenoid valve becomes significant, requiring the above approximation to be refined. The delay in opening the solenoid valve depends on the rate at which current through the coil builds--i.e., voltage--while the closing delay is essentially fixed. Accordingly, a more useful relationship is expressed as follows:
Fuel Flow=(PW+Offset (V)).times.f.times.C.
Experience has shown that, in order to obtain consistent delays or "offsets", it is desirable to initially allow the injector current to build to a high value until the valve begins to move and then reduce the injector current to a lower or "holding" value for the remainder of the solenoid pulse to avoid excessive heating of the injector winding. At the end of the pulse, the injector current is then rapidly decayed by allowing a large induced voltage to develop in the winding of the solenoid.
In multipoint fuel injection systems, the above control sequence is typically accomplished by operating a power transistor into saturation until the desired peak current is reached, turning the transistor off briefly until the injector current decays to the desired "hold" value, and then maintaining the "hold" current by operating the transistor in its active region until the transistor is turned off at the end of the pulse. However, because of high transistor power dissipation in the active "hold" region, relatively large heat sinks are required. While such heat dissipation is managable in multipoint fuel injection systems where the individual injectors are fairly small, in throttle body fuel injection systems where a single large injector is utilized to meter fuel flow, the increased force required to move the injector valve requires significantly higher current levels, resulting in commensurately higher levels of heat. For example, the current requirements of a multipoint fuel injector typically are I-peak/I-hold of 2A/0.5A respectively, while those of an exemplary single point fuel injector are 6-8.8A/1.5A. Thus, in automotive throttle body fuel injection applications, the active region method of maintaining "hold" currents may become impractical due to the difficulty of dissipating the increased levels of heat generated.
As a potential solution to this problem, it has been proposed that the desired injector holding current be maintained by rapidly switching the power transistor on and off at an appropriate switching frequency. This approach is taught, for example, in Schultzke et al., U.S. Pat. No. 4,180,026, assigned to Robert Bosch GmbH. The disadvantage of this approach, however, is that the induced voltage which develops in the injector winding during the periods when the transistor is turned off causes the injector current to decay rapidly during shut-off, thus requiring a relatively high switching frequency to avoid injector "chatter" and maintain the desired average holding current level. High switching frequencies can, however, cause switching dissipation and radio frequency interference problems. In addition, performance of the injectors may be less than optimal, particularly at small injector pulse widths.
Accordingly, it is the primary object of the present invention to provide an improved control circuit for an electronic fuel injection system.
In addition, it is an object of the present invention to provide an improved electronic fuel injection control circuit which produces highly consistent delays in the actuation of the fuel injector valve.
Furthermore, it is an object of the present invention to provide an electronic fuel injection control circuit which is capable of precisely controlling relatively large single port fuel injectors without creating a heat dissipation problem.
It is also an object of the present invention to provide an electronic fuel injection control circuit which provides improved linearity in the operation of the fuel injector.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :